The present invention relates to a semiconductor laser device particularly in which a plurality of semiconductor laser elements including a semiconductor laser element that is easily deteriorated upon a high-temperature operation are mounted on the same semiconductor submount, and a method of manufacturing the same.
Since a material of each semiconductor laser element is different in a semiconductor laser device in which a plurality of semiconductor laser elements with different emission wavelengths are included in one package, a temperature characteristic of one semiconductor laser element becomes worse than those of other semiconductor laser elements (for example, see Patent Literatures 1, 2). Specifically, when temperature is changed by 1° C., for example, characteristics such as light-emitting efficiency and an oscillation threshold are largely changed in one semiconductor laser element in comparison with those of other semiconductor laser elements.
More specifically, for example, in a semiconductor laser device in which an infrared laser element with a wavelength in a 780 nm band and a red laser element with a wavelength in a 650 nm band are die-bonded on a same silicon submount, a temperature characteristic of the red laser element is poorer than that of the infrared laser element. Therefore, it is necessary to place a light-emitting section of the red laser element (a portion on a pn-junction surface) closer to the silicon submount to reduce heat resistance and an operation current upon a high-temperature operation so that breakdown and deterioration of the laser are prevented.
Furthermore, when the aforementioned semiconductor laser device, which is a two-wavelength semiconductor laser device, is incorporated into a light pickup, light reflected on an optical disc is made incident to and reflected on an end surface of the infrared laser element. To prevent the light reflected on the end surface of the infrared laser element from returning to a light pickup optical system, an emission point of the infrared laser element is preferably positioned at a center in a chip-height direction. At this time, an N-type side of the infrared laser element is preferably die-bonded on the silicon submount to prevent deterioration of the temperature characteristic of the infrared laser element where possible.
FIG. 5 is a schematic cross-sectional view showing a substantial part of a conventional two-wavelength semiconductor laser device viewed from its front. FIG. 6 is a schematic view showing a cross-section along line VI—VI in FIG. 5. That is, FIG. 6 is a schematic cross-sectional view of the two-wavelength semiconductor laser device viewed from its side.
As shown in FIG. 5, the two-wavelength semiconductor laser device includes a silicon submount 116 with a photodiode, a red laser chip 114 and an infrared laser chip 115 die-bonded on this silicon submount 116 with a photodiode.
The red laser chip 114 is a 650-nm band laser chip, and has a structure with a junction-side down constituted by a P-type layer 109 having a thickness of 5–6 μm and an N-type layer 110 having a thickness of about 110 μm to improve heat dissipation and reduce heat resistance.
The infrared laser chip 115 is a 780-nm band laser chip and is constituted by an N-type layer 111 having a thickness of about 40 μm and a P-type layer 112 having a thickness of about 70 μm so that an active layer (a layer on a junction surface of the P-type layer 112 and the N-type layer 111) is positioned at a center in a chip height direction to prevent return light.
The red laser chip 114 and the infrared laser chip 115 are die-bonded on the silicon submount 116 with a photodiode so that their polarities on the side of the silicon submount 116 with a photodiode are different from each other. Therefore, an insulating film 105 made of SiO2 or the like is formed so as to cover a surface (die-bonded surface) of the silicon submount 116 with a photodiode so that the red laser chip 114 and the infrared laser chip 115 are electrically isolated.
Furthermore, an Al layer 106, TiW layer (not shown) as a barrier metal, Au layer 107 and AuSn layer 108 are successively formed on the insulating film 105. The Al layer 106 is provided to have an ohmic contact with Si, and the Au layer 107 is provided to increase adhesion to the AuSn layer 108. Furthermore, the AuSn layer 108 is provided for a junction with Au electrodes on rear surfaces of the laser chips. That is, the red laser chip 114 and the infrared laser chip 115 are die-bonded on the AuSn layer 108.
The silicon submount 116 with a photodiode is constituted by an N+-type substrate 101 and an N− epitaxial layer 102 formed on this N+-type substrate 101. As shown in FIG. 6, a P-type diffusion region 103, which is to serve as a light-receiving portion of the photodiode, is disposed on the N− epitaxial layer 102.
FIG. 7 is a schematic plan view showing the two-wavelength semiconductor laser device viewed from the die-bonded surface side to show the P-type diffusion region 103 of the silicon submount 116 with a photodiode. To simplify the figure, the red laser chip 114 and the infrared laser chip 115 are not shown in FIG. 7.
When light emitted from end surfaces of the red laser chip 114 and the infrared laser chip 115 on the P-type diffusion region 103 side is made incident to the P-type diffusion region 103, which is a light-receiving portion, an output can be obtained depending on an optical power of the light. Consequently, an output (laser optical power) of the light emitted from end surfaces of the red laser chip 114 and the infrared laser chip 115 on the opposite side of the P-type diffusion region 103 is monitored. It is noted that there is one P-type diffusion region 103, which is a light-receiving portion since the red laser chip 114 and the infrared laser chip 115 do not need to light simultaneously.
Patent Literature 1
Japanese Patent Laid-Open Publication No. 2000-222768
Patent Literature 2
Japanese Patent Laid-Open Publication No. 2001-189516
However, in the conventional two-wavelength semiconductor laser device, heat dissipation is poor since the insulating film 105 is disposed immediately under electrodes of the red laser chip 114 and the infrared laser chip 115. That is, heat of the red laser chip 114 and the infrared laser chip 115 cannot be efficiently dissipated. As a result, deterioration or failure easily occurs particularly in the red laser chip 114 upon a high-temperature operation.
Since a 650-nm band red laser is less efficient than a 780-nm band infrared laser under a high temperature atmosphere due to their difference in materials and bandgap structure, a current is increased, or deterioration or failure occurs upon a high-temperature operation.